The present invention relates to drive shafts, and in particular to so called flexible drive shafts for transmitting rotary drive used for example in automotive applications and specifically, but not exclusively for powered vehicle seat adjustment.
Flexible drive shafts, known as ‘flex shafts’, are used in a number of automotive applications to transmit rotary motion from a drive or motor to a remote location (for example a remote gearbox) located away from the drive motor where it is required. In particular, for example, flex shafts are used in powered vehicle seats to drivingly interconnect a motor located centrally underneath the seat to the seat track fore and aft slide mechanisms located towards either side of the base of the seat. The seat track fore and aft slide mechanisms providing for fore and aft adjustment and movement of the seat. Flex shafts may also be used to interconnect other driven seat adjustment mechanisms with a remotely located motor. It will be appreciated that often, for space, noise, or other reasons, it is desirable for the motor unit generating rotary powered drive to be located at a remote location away from the driven adjustment mechanisms. Furthermore in recent vehicle seat arrangements it is proposed to conveniently locate all of the drive motors together in a single unit centrally located on the base of the seat, with flex shafts transmitting the drive from the motors to the various adjustment mechanisms (for example fore and aft powdered seat slides, seat height adjusters, seat back tilt adjusters) of the seat and which are located around the vehicle seat.
Flex shafts conventionally comprise a central rotatable flexible core shaft through which the rotary drive is transmitted. The flexible core shaft is swaged (or otherwise joined) onto the motor output drive shaft (or other drive) at one end of the shaft, and the driven element input shaft (eg. Adjuster box) at the other end of the shaft. The core shaft is located and encased within an outer (stationary) sleeve which protects the centrally rotating core shaft. The outer sleeve may be flexible or stiff, and may be bent or formed along its length into any required shape or path to interconnect between the motor and driven element with the inner flexible core located therein. It should be noted that the term ‘flex shaft’, or flexible drive shaft, refers to the inner core shaft which is flexible and can be configured into any required path, and does not require the outer sleeve or entire device shaft assembly to be flexible. To prevent the inner core flexible drive shaft for contacting and jamming against the inside of the outer sleeve as the inner shaft rotates, in particular when the shaft is bent, and to reduce noise lubrication grease is packed inside the outer sleeve to fill the annular space between the flexible core shaft and outer sleeve.
In operation the core shaft of flex drive may typically rotate at a range of speeds up to, and beyond, for example 5000 rpm in automotive applications. Whilst flex shafts adequately transmit rotary drive such shafts can, in particular at and over such a wide range of operating speeds as may be required in automotive applications, generate unacceptable noise levels for interior vehicle applications where they is a constant desire to reduce noise from such powered adjustment mechanisms, and a desire for silent operation. Furthermore the noise generated from conventional assembled production flex shafts is not consistent. Substantially identical assembled flex shafts often produce differing amounts of noise, and there is also considerable noise variation over the range of operating speeds.
In addition, installation and assembly of conventional flex shafts can sometimes be difficult and messy. High accuracy assembly tolerances are also required to reduce eccentric running. Induced stresses within the flex shaft due, for example, to assembly and manufacturing misalignment can also be a problem.
It is therefore desirable to provide an improved drive shaft which addresses the above described problems and/or which offers improvements generally.